Pentafluoroethane (HFC-125), difluoromethane (HFC-32), and other hydrofluorocarbons (HFC) are widely used as important alternatives for chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), and other substances that may destroy the ozone layer. Hydrofluorocarbons have various applications, such as heat transfer media, refrigerants, foaming agents, solvents, cleaning agents, propellants, and fire extinguishers, and are consumed in large amounts.
However, these hydrofluorocarbons are potent global-warming substances. Many people are concerned that their diffusion may affect global warming. To combat this, hydrofluorocarbons are collected after being used; however, not all of them can be collected, and their diffusion due to leakage, etc., cannot be disregarded. In particular, for use in refrigerants, heat transfer media, etc., although substitution of hydrofluorocarbons with CO2 or hydrocarbon-based substances has been studied, CO2 refrigerants have many difficulties in reducing comprehensive greenhouse gas emissions, including energy consumption, because of the requirement of large equipment due to the low efficiency of the CO2 refrigerants, and hydrocarbon-based substances have safety problems due to their high flammability.
Recently, hydrofluoroolefins with a low warming potential are attracting attention as substances that can solve these problems. “Hydrofluoroolefin” is a generic name for unsaturated hydrocarbons containing hydrogen and fluorine. Most of them are obtained by dehydrohalogenation of corresponding alkanes. For example, as a typical method for producing 2,3,3,3-tetrafluoropropene (HFO-1234yf), which is a hydrofluoroolefin, a method of eliminating HF from 1,1,1,2,3-pentafluoropropane (HFC-245eb) or 1,1,1,2,2-pentafluoropropane (HFC-245cb) is known. Since HFO-1234yf obtained by this method is in the form of a mixture with HF, it is necessary to somehow remove the HF. The simplest method of removing acid from a mixed gas of hydrofluoroolefin and acid is to absorb the acid with water. In this method, however, mist and water corresponding to vapor pressure are inevitably mixed into hydrofluoroolefins. There are various other sources of water, such as moisture contained in the starting materials, moisture produced from the catalyst, and moisture remaining in the equipment. Moisture contained in hydrofluoroolefins, which are finished products, affects their stability, device corrosiveness, refrigerant capability, etc., and is therefore one of the most important factors of quality control. The method for removing moisture is a particularly important technique.
As the method for removing moisture from hydrofluoroolefins, a method using a molecular sieve (e.g., zeolite) as a water absorbent is known. For example, Patent Literature 1 discloses a method for drying a fluid of a fluoropropene, such as HFO-1234yf, by passing it through zeolite. However, a large packed column is required to treat fluoropropenes with a low moisture content using a water absorbent, which causes poor treatment efficiency. In addition, in the method using an absorbent, there are problems that it is necessary to regularly stop the equipment to recover the performance of the absorbent and exchange the absorbent, which leads to low productivity and necessitates the use of two series of equipment. Moreover, a large amount of industrial waste is generated during the exchange of the absorbent. Furthermore, depending on the hydrofluoroolefins to be treated, they may be absorbed to the absorbent, presumably inhibiting the absorption of water that should be essentially removed.
For the above reasons, there is a demand for more efficient methods for removing moisture from various fluorine-containing compounds, such as hydrofluoroolefins.